This invention relates to an optically recording and/or reproducing apparatus which records and/or reproduces an information such as digital information, video information, and audio information by projecting a light flux emitted from a light source such as a laser onto a rotating disc-shaped optical record medium.
In case of recording the information such as digital information, video information, and audio information on a disc-shaped record medium, usually an optically recording method is used. In such an optically recording method, a disc coated with a resist film or a metal thin film, etc., on a glass plate or a plastic plate is used as a record medium, an intensity of a laser beam of a high-power energy is modulated with the information to be recorded, and the laser beam thus modulated is projected onto the disc. The film on the disc is partly melted and evaporated and in this manner the information is recorded on the disc as a fine pit construction. In order to reproduce the information from the disc having such pit construction, the laser beam having a low-power energy and a constant intensity is made incident upon the disc and then a light flux reflected by or transmitted through the disc is detected. In such an optically recording and/or reproducing apparatus, since the laser light beam which is converged by an objective lens is made incident upon the disc, a so-called focussing control should be effected such that the objective lens is moved in its optical axial direction so as to be able to form a small spot with a diameter of about 0.8 .mu.m on the disc. Further, it is necessary to perform a so-called tracking control such that the light beam is moved in a disc radial direction so that the spot always traces a center of an information track. Usually, as the focussing control mechanism, a moving coil type is used. In the focussing control mechanism of moving coil type, a magnetic field passing through a coil which is wound around an objective lens holder is generated, and the lens holder and thus, the objective lens are deviated in the optical axial direction by supplying to the coil, a compensating signal corresponding to a focussing error, so that a distance between the disc and the objective lens is made always constant. In a usual tracking control mechanism, a galvanomirror including a driving coil is arranged in a light path between the laser light source and the objective lens, and a compensating signal corresponding to a tracking error is supplied to the coil to rotate the mirror, so that the laser light spot is shifted in the disc radial direction according to aforesaid signal.
In the tracking control system using such a galvanomirror, the light beam impinges upon the objective lens at an inclined angle, and therefore aberrations of the objective lens must be corrected strictly over its whole aperture, so that there is a drawback that a design of the lens is difficult and a cost of the lens is very expensive. There are further drawbacks that the galvanomirror itself is very expensive, a miniaturization is limited, and a reliability of this system is rather low.
In the optically recording and/or reproducing apparatus of the type mentioned above, there is further provided a radial driving system for moving an optical system including the objective lens in the disc radial direction, but a backlash of this driving system results in that in case of recording, a track could not be recorded at a desired position with a given constant pitch and in case of reproducing, the track could be hardly traced and the tracking mechanism would be subjected to more severe requirements. Moreover, when a disc having such a pitch deviation reproduced, an interference between neighboring tracks is undesirably occurs. In order to prevent such an interference, it is necessary to take a sufficiently wide track pitch or to use a complicated and expensive mechanism having an extremely high feeding precision in the radial direction. It is apparent that the wider the track pitch is, the lower a recording density becomes and thus, one of the significant characteristics of such an optically recording and/or reproducing system is lost. Moreover, in case of using such an optically recording and/or reproducing apparatus as a digital data memory, that is, a terminal device of a computer, it is necessary for the apparatus to be small in size and simple in construction.
In the above mentioned digital data memory or reproducing apparatus for the video information and the audio information, a random access mechanism for projecting the laser light beam onto any desired position on the disc is provided. In order to shorten an access time so as to effect the random access fast, the precision of the random access might be decreased. Contrary to this in order to make the precision high, the access speed might be decreased. Especially, in case of using the disc having a very high record density and the very narrow pitch between successive tracks, it is very difficult to perform the precise random access at a high speed.
Further, in case of recording or reproducing the information on or from the disc, it is necessary to move the objective lens linearly in the disc radial direction substantially over its radius. However, the known tracking system could not satisfy such a requirement to a full extent.